Macmillan have a useful guide here: Cannabis oil and cancer

We are currently a long way from having a useful anti-cancer drug derived from cannabis, and there is some danger in attempting self medication.  I'm not an expert but I see two major problems:

1. Many people are under the impression that cancer is one illness. But in fact, it is an all encompassing name for many different illnesses with some common characteristics. Each form of cancer requires different treatment, and what works for some cancers doesn't work for others. We know that specific chemo drugs only work against a limited range of cancers, and there's no reason to think that drugs derived from cannabis will be any different.  It is therefore pure chance whether or not buying any form of CDB is going to be effective against a particular tumour in a particular patient, with the odds stacked heavily against.

2. Late stage cancers evolve rapidly.  It is a known feature of many cancers that in their late stages the cells mutate rapidly, and hence they evolve resistance against particular drugs that were once controlling them.  What happens is that the susceptible cells are killed off, leaving behind non-susceptible cells which can proliferate without any control.  In the Macmillan report above, there is a sentence that says "in some situations, encourage cancer cells to grow" and I wonder if this is what's happening here. This frightens me. It means that randomly buying CBD oil in the hope that it may do some good could end up doing more harm.  

I think it likely that at some future time there will be a number of useful drugs derived from cannabis. However, it is highly unlikely that these drugs will be effective against all cancers; more likely, they will somewhat effective against a highly limited range of cancers, and will end up in the pharmacopeia alongside existing chemo drugs.  What won't happen is that there will be a game changing drug that will defeat all cancers.  

Pulling all this together, I am very wary of CBD oil as an anti-cancer treatment, and I would never use it to self-medicate (or for a loved one) for fear of it making things worse.